Moore’s starting point here is a famous section of Kant’s Critique of Pure Reason entitled “Refutation of Idealism” in which he offers a “proof” of “the existence of objects in space outside me.” Moore will have little to say about the details of Kant’s proof (if you are curious, you can find the heart of it quoted below), but he will offer his own.

Moore’s paper has a very simple structure, and I will merely point you to it. You should, of course, think about his proof; but that is presented only very late in the paper, and you should think also about his preliminary discussions and the issues they raise.

•  After a few introductory paragraphs (pp. 130-131), Moore’s preliminary discussions concern the idea of externality. The first of these (pp. 131-139) focuses on the idea of “things which are to be met with in space.” This is a phrase Kant uses to explain the idea of externality at one point (though not in his proof); and, although Moore doesn’t really accept this as adequate, the idea proves useful in presenting his own proof.

•  Moore’s second preliminary discussion (pp. 139-143) concerns the idea of externality itself and, eventually, its relation to the first idea.

•  By the end of these preliminary discussions, it will become clear how Moore thinks he can prove the existence of an external. He offers his proof on p. 144 and the rest of the paper is devoted to a discussion of it. Part of this is a proof of the past existence of an external world (p. 145). Before this he gives reasons for thinking his proof really is one (when you read it, you’ll see why this might be necessary); and, after his second proof, he considers why people might find the two proofs unsatisfactory.

Kant’s proof from B 275-276 of the Critique of Pure Reason, Norman Kemp Smith (tr.) (London: Macmillan, 1929), p. 245. (It is standard to cite passages in this work by their pages in the two editions Kant published, using letters to identify the edition; Kant’s refutation of idealism appeared only the second edition, labeled B.)

THESIS

The mere, but empirically determined, consciousness of my own existence proves the existence of objects in space outside me.

Proof

I am conscious of my own existence as determined in time. All determination of time presupposes something permanent in perception. This permanent cannot, however, be something in me, since it is only through this permanent that my existence in time can itself be determined.¹ Thus perception of this permanent is possible only through a thing outside me and not through the mere representation of a thing outside me; and consequently the determination of my existence in time is possible only through the existence of actual things which I perceive outside me. Now consciousness [of my existence] in time is necessarily bound up with consciousness of the [condition of the] possibility of this time-determination; and it is therefore necessarily bound up with the existence of things outside me, as the condition of the time-determination. In other words, the consciousness of my existence is at the same time an immediate consciousness of the existence of other things outside me.

¹ [As stated by Kant in the Preface to B (above, p. 36 n.), this sentence should be altered as follows: “But this permanent cannot be an intuition in me. For all grounds of determination of my existence which are to be met with in me are representations; and as representations themselves require a permanent distinct from them, in relation to which their change, and so my existence in the time wherein they change, may be determined.”]